1. Field of the Invention
This invention relates to a process for enhancing anaerobic biohydrogen production, which includes adding into a reactor an organic waste containing hydrogen-producing anaerobic bacteria, a first substrate, and a carrier that permits the hydrogen-producing anaerobic bacteria to adhere thereto and to grow thereon, so as to form a mixture; acclimating the mixture at a first agitation speed ranging from 5 to 60 rpm, so that the hydrogen-producing anaerobic bacteria adhere to and proliferate on the carrier, followed by the formation of granular biomasses constituted of the hydrogen-producing anaerobic bacteria within the acclimated mixture; and feeding a second substrate into the reactor at a second agitation speed ranging from 5 to 60 rpm, so that the content of the granular biomasses is increased while the second substrate is anaerobically fermented by the hydrogen-producing anaerobic bacteria in the granular biomasses to result in the production of hydrogen.
2. Description of the Related Art
Recently, hydrogen has attracted wide attention and is regarded as one of the important energy sources of the future because it has the advantages of cleanness, non-polluting, good recyclability, and high heat energy (D. B. Levin et al. (2004), International Journal of Hydrogen Energy, 29:173-185).
Hydrogen production methods can be classified into three main types, namely, the thermochemical method, the electrochemical method, and the biological method.
The thermochemical method includes steam reforming of methane, coal gasification, partial oxidation of hydrocarbon compounds, etc. These hydrogen production methods have been industrialized, but since consumption of a large amount of mineral resources and energy is required and pollutants harmful to the environment are generated during the production process, they do not contribute to the development of energy resources.
Although the electrochemical method (e.g., electrolysis, photoelectrolysis, etc.) does not engender environment pollution, and the purity of hydrogen produced thereby is relatively high, it still has problems of low efficiency, high energy consumption, unsatisfactory electrode stability, etc., which remain to be solved (D. Das et al. (2001), International Journal of Hydrogen Energy, 26:13-28).
In contrast, the cost required to produce hydrogen using biodegradation is relatively low, and is therefore more advantageous than chemical hydrogen production methods. The biological hydrogen production method includes: (1) biophotolysis of water, which is the use of photo-induction to encourage algae or cyanobacteria to decompose water so as to result in the production of hydrogen; (2) photo-fermentation, which is the use of photosynthetic bacteria to decompose organic matters so as to result in the production of hydrogen; and (3) dark-fermentation, which is the use of anaerobic bacteria to decompose organic matters to result in the production of hydrogen. Biophotolysis of water and photo-fermentation require a higher reaction free energy, whereas dark-fermentation does not require a light source and can utilize various organic matters as substrates. Therefore, in industrial applications, dark-fermentation hydrogen production techniques have been extensively used to process organic waste.
Use of anaerobic bacteria to process organic matter-containing wastewater or waste (e.g., kitchen leftovers, wastes of agricultural or energy crops, and wastewater or waste of organic industries, etc.) not only can produce hydrogen for use as energy, organic acids can also be produced for re-use. Besides, expenses associated with processing of various kinds of sludge and organic wastewater or waste can be reduced.
The design of a conventional anaerobic fermentative reaction system primarily attempts to shorten the hydraulic retention time (HRT) of system operation so as to increase the organic loading rate (OLR) of the system, while trying to enhance the degradation rate by increasing the concentration of hydrogenogenic bacteria or the activity of sludge, thereby achieving the objective of enhancing the efficiency of biological wastewater treatment (G. Lettinga et al., (1980), Biotechnology and Bioengineefing, 22:699-734).
Conventional continuous stirred tank reactors (CSTR) have extensive applications in the study of anaerobic fermentative hydrogen production because of simple system operation and good mixing capability. Regarding studies in this respect, reference can be made to, for instance, C. Y. Lin et al., (1999), Journal of Chemical Technology and Biotechnology, 74:498-500; 0. Mizuno et al., (2000), Bioresource Technology, 73:59-65; C. C. Chen et al., (2001), Appl Microbiol Biotechnol., 57:56-64; H. H. P. Fang et al., (2002), Biotechnology and Bioengineering, 78:44-52; J. I. Horiuchi et al., (2002), Bioresource Technology, 82:209-213; H. Liu et al., (2002), Water Science and Technology, 47(1): 153-158; and J. S. Chang et al., “Using Environmental Biological Techniques to Produce a Clean Energy—Hydrogen,” Chemical Engineering, Volume 49, No. 6, 85-104, December, 2002.
However, these prior studies show that when the continuous stirred tank reactor is used to conduct an anaerobic fermentative hydrogen production reaction under a relatively low hydraulic retention time, wash-out often occurs between the fed substances and microorganisms so that concentration of biomasses containing hydrogenogenic bacteria drops, thereby reducing the hydrogen production efficiency. For example, Chang et al. discovered that, when the continuous stirred tank reactor operates under the condition that HRT≧4 hr., wash-out of the hydrogenogenic bacteria-containing biomasses in the tank reactor will occur, which is detrimental to the process of anaerobic fermentative hydrogen producing reaction [Chang et al. (2002), supra].
In TW 200417533 of Lai et al., there is disclosed a process for anaerobic hydrogen production, which includes the following steps: (1) crushing waste into granular particles with a length and width less than 1 mm and mixing the same with water; (II) pre-treatment and formulation of seeding material; (III) anaerobic fermentation to produce hydrogen; (IV) anaerobic fermentation to produce methane gas; and (V) purification of discharged gas from the hydrogen production fermentation tank. In step (III), if a batch reaction is conducted, the rotational speed of the reactor is between 25 and 35 rpm. On the other hand, if a continuous reaction is conducted, the rotational speed of the reactor is between 30 and 100 rpm.
In addition, Lee et al. disclose a biohydrogen production technique, in which spherical activated carbon and cylindrical activated carbon are used as carriers, and are filled into a fixed bed reactor for hydrogenogenic bacteria to adhere thereto and to grow thereon so as to form a biofilm. The aforesaid technique can effectively maintain the concentration of hydrogenogenic bacteria in the reactor and suppress occurrence of wash-out so that anaerobic fermentative hydrogen reaction can be stably conducted under operating conditions of a low hydraulic retention time and a high organic loading rate (Lee et al., Continuous Hydrogen Fermentation Using a Biofilm Reactor, Proceedings of the 7th Biochemical Engineering Symposium, Jun. 28-29, 2002).
According to this prior study, when a fixed bed reactor having a 0.3 L or 3 L working volume is used to conduct an anaerobic fermentative hydrogen production reaction, hydrogen can be produced stably at a hydraulic retention time of 1 hour. The hydrogen production rate (HPR) is approximately 1.21 L/h/L, whereas the hydrogen content (H2 content) can reach approximately more than 30%. However, when the hydraulic retention time is lowered to 0.5 hour, the anaerobic fermentative reaction system will show signs of instability, thereby resulting in a lowered hydrogen production rate.
Therefore, there still exists a need to develop a new anaerobic biohydrogen production technique, which can achieve an excellent hydrogen production efficiency even under operating conditions of a low hydraulic retention time and a high organic loading rate.